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511.
Catharina Johanna Maria Philippart Jolanda Martine van Iperen Gerhard Cornelis Cadée Alain François Zuur 《Estuaries and Coasts》2010,33(2):286-294
Analyses of long-term field observations (1974–2007) on chlorophyll-a concentrations in the western Wadden Sea showed no long-term trends in the timing of the wax and wane of phytoplankton spring
blooms. There is weak evidence, however, that the height of the autumn bloom has decreased since the early 1990s. This fading
of the autumn bloom may have had consequences for the carbon transfer to higher trophic levels, currently hampering primary
consumer species that mostly rely on food supply during late summer. Current and other findings suggest a shortening of the
growing season due to the fading of the autumn bloom in the Wadden Sea and a lengthening of the growing season due to an advancement
of the spring bloom in the North Sea. These regionally different changes in seasonality may have contributed to the coinciding
decrease in bivalve filtering capacity in the western Wadden Sea and the large-scale offshore shift of juvenile plaice from
the Wadden Sea to the adjacent North Sea. 相似文献
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In a context of increased demand for food and of climate change, the water consumptions associated with the agricultural practice of irrigation focuses attention. In order to analyze the global influence of irrigation on the water cycle, the land surface model ORCHIDEE is coupled to the GCM LMDZ to simulate the impact of irrigation on climate. A 30-year simulation which takes into account irrigation is compared with a simulation which does not. Differences are usually not significant on average over all land surfaces but hydrological variables are significantly affected by irrigation over some of the main irrigated river basins. Significant impacts over the Mississippi river basin are shown to be contrasted between eastern and western regions. An increase in summer precipitation is simulated over the arid western region in association with enhanced evapotranspiration whereas a decrease in precipitation occurs over the wet eastern part of the basin. Over the Indian peninsula where irrigation is high during winter and spring, a delay of 6?days is found for the mean monsoon onset date when irrigation is activated, leading to a significant decrease in precipitation during May to July. Moreover, the higher decrease occurs in June when the water requirements by crops are maximum, exacerbating water scarcity in this region. A significant cooling of the land surfaces occurs during the period of high irrigation leading to a decrease of the land-sea heat contrast in June, which delays the monsoon onset. 相似文献
515.
A case study of land cover change (1950-2003) and runoff in a Mediterranean catchment 总被引:2,自引:0,他引:2
Dennis M. Fox Emmanuelle WitzViolaine Blanc Cécile SouliéMarc Penalver-Navarro Alain Dervieux 《Applied geography (Sevenoaks, England)》2012,32(2):810-821
Mediterranean environments have been subject to major land cover change since the end of the second world war. Housing, agricultural activities, forests, green spaces and other land uses have shifted due to urbanisation and tourism. These changes influence runoff, and municipal authorities often cannot estimate the net impact of complex land cover transitions. During this period, elected representatives have become increasingly sensitive to the risks of flooding and have implemented a number of channel management strategies. The main objective of this case study was to analyse the impact of land cover change on total storm runoff between 1950 and 2003 in a Mediterranean catchment near St Tropez, France. A secondary objective was to compare these changes to the impacts of channel management on bankfull discharge. Aerial photographs were used to classify land cover in 3 urban categories, vineyards and bare soil, forests, and green spaces. Stream discharge was estimated using a distributed event based total runoff approach. After validating the model for a large winter event (114 mm) for 1982, runoff was calculated for the same event for 1950 and 2003. Land cover changes occurred mainly in the alluvial plain area. Total gauge catchment urban area increased from 30.1 ha to 393.8 between 1950 and 2003 at the expense mainly of agricultural land, but this was compensated in part by an increase in grassed area. Some of the loss in vineyards was replaced by clearing forested land on the first hills close to the plain. Bank stabilisation and channel maintenance since the 1980’s reduced surface roughness and increased channel area, thereby greatly increasing bankfull discharge. While the impact of urbanisation on runoff was small, channel management effects increased bankfull discharge substantially. Flood damage from extreme events was not studied here. 相似文献
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The stability of the thermohaline circulation of modern and glacial climates is compared with the help of a two dimensional ocean—atmosphere—sea ice coupled model. It turns out to be more unstable as less freshwater forcing is required to induce a polar halocline catastrophy in glacial climates. The large insulation of the ocean by the extensive sea ice cover changes the temperature boundary condition and the deepwater formation regions moves much further South. The nature of the instability is of oceanic origin, identical to that found in ocean models under mixed boundary conditions. With similar strengths of the oceanic circulation and rates of deep water formation for warm and cold climates, the loss of stability of the cold climate is due to the weak thermal stratification caused by the cooling of surface waters, the deep water temperatures being regulated by the temperature of freezing. Weaker stratification with similar overturning leads to a weakening of the meridional oceanic heat transport which is the major negative feedback stabilizing the oceanic circulation. Within the unstable regime periodic millennial oscillations occur spontaneously. The climate oscillates between a strong convective thermally driven oceanic state and a weak one driven by large salinity gradients. Both states are unstable. The atmosphere of low thermal inertia is carried along by the oceanic overturning while the variation of sea ice is out of phase with the oceanic heat content. During the abrupt warming events that punctuate the course of a millennial oscillation, sea ice variations are shown respectively to damp (amplify) the amplitude of the oceanic (atmospheric) response. This sensitivity of the oceanic circulation to a reduced concentration of greenhouse gases and to freshwater forcing adds support to the hypothesis that the millennial oscillations of the last glacial period, the so called Dansgaard—Oeschger events, may be internal instabilities of the climate system. 相似文献
518.
On the Impact of Anthropogenic Heat Fluxes on the Urban Boundary Layer: A Two-Dimensional Numerical Study 总被引:2,自引:1,他引:1
Andrea Krpo Francisco Salamanca Alberto Martilli Alain Clappier 《Boundary-Layer Meteorology》2010,136(1):105-127
The heat generated in buildings and the manner in which this heat is exchanged with the ambient environment can play an important
role in urban climate. Recent studies have shown that anthropogenic heat from air-conditioning facilities can increase the
exterior ambient temperature and should be taken into account for a more complete urban heat island (UHI) mitigation study.
For this purpose, the first part of the present work is focused on the coupling of a new building energy model (BEM) and an
urban canopy parameterisation (UCP). The new scheme is implemented in a finite volume mesoscale model (MM) and tested in a
two-dimensional (2D) configuration of a city over flat terrain. A sensitivity study is performed with respect to different
parameters in order to test the simulation system and enhance the understanding of the possible impacts of the BEM on the
exterior microclimate. 相似文献
519.
Analyses of zircon grains from the Queureuilh Quaternary tephras (pumice) provide new information about their pre-eruptive history. U-Pb dating was performed in situ using two methods: SHRIMP and LA-MC-ICPMS equipped with a multi-ion counting system. Both methods provided reliable 207Pb/206Pb and 206Pb/238U ratios as well as U and Th abundances required for U-Pb Concordia intercept age determination, after initial 230Th disequilibrium correction. The new LA-MC-ICPMS method was validated by dating a reference zircon (61.308B) and zircons from a phonolitic lava dated independently with the two techniques. A time resolution of about 20 kyr for 1 Ma zircon crystals was achieved for both methods.The clear euhedral zircon population from Queureuilh tephras is quite complex from several points of view: (1) some grains are reddish or yellowish while others are colorless; (2) the U and Th composition changes by more than an order of magnitude and Th/U is generally high (∼1-2); (3) there are three discrete ages recorded at 2.35 ± 0.04, 1.017 ± 0.008 and 0.640 ± 0.010 Ma.From the previously determined 40Ar/39Ar age at 0.571 ± 0.060 Ma [Duffell H. (1999) Contribution géochronologique à la stratigraphie volcanique du Massif des Monts Dore par la méthode 40Ar/39Ar. D.E.A. Univ. Clermont-Ferrand, 56 p.], the discontinuous zircon age populations, the color of the grains and their composition, we favor the following model as explanation: The oldest, less numerous group of reddish zircons represents xenocrystic grains resulting from assimilation of the local material during magma ascent. A primitive magma chamber, perhaps deep in crustal level, was formed at 1.0 Ma. The related magma, previously characterized by high Th/U ratio (2.2 ± 1.1), underwent rejuvenation during ascent to a new chamber at shallow depth and/or during injection of more mafic magmas. During this stage, at 0.64 Ma, the colorless zircon grains of lower Th/U ratio (1.3 ± 0.5) crystallized. This last stage defined the magma residence time of 70 kyr prior to eruption dated by the 40Ar/39Ar method. However, if the primitive magma is considered, the magma residence time as a whole from this first stage reached 446 kyr.In the light of the complex history of such magmas, which commonly involves recycling of zircon grains that precipitated tens to hundreds of kyr earlier than eruptions, the use of Zr concentration in geochemical modeling of whole rock compositional data can be problematic. 相似文献
520.
Laurent Koechlin Denis Serre Paul Deba Roser Pelló Christelle Peillon Paul Duchon Ana Ines Gomez de Castro Margarita Karovska Jean-Michel Désert David Ehrenreich Guillaume Hebrard Alain Lecavelier des Etangs Roger Ferlet David Sing Alfred Vidal-Madjar 《Experimental Astronomy》2009,23(1):379-402
The Fresnel Interferometric Imager has been proposed to the European Space Agency (ESA) Cosmic Vision plan as a class L mission.
This mission addresses several themes of the CV Plan: Exoplanet study, Matter in extreme conditions, and The Universe taking shape. This paper is an abridged version of the original ESA proposal. We have removed most of the technical and financial issues,
to concentrate on the instrumental design and astrophysical missions. The instrument proposed is an ultra-lightweight telescope,
featuring a novel optical concept based on diffraction focussing. It yields high dynamic range images, while releasing constraints
on positioning and manufacturing of the main optical elements. This concept should open the way to very large apertures in
space. In this two spacecraft formation-flying instrument, one spacecraft holds the focussing element: the Fresnel interferometric
array; the other spacecraft holds the field optics, focal instrumentation, and detectors. The Fresnel array proposed here
is a 3.6 ×3.6 m square opaque foil punched with 105 to 106 void “subapertures”. Focusing is achieved with no other optical element: the shape and positioning of the subapertures (holes
in the foil) is responsible for beam combining by diffraction, and 5% to 10% of the total incident light ends up into a sharp
focus. The consequence of this high number of subapertures is high dynamic range images. In addition, as it uses only a combination
of vacuum and opaque material, this focussing method is potentially efficient over a very broad wavelength domain. The focal
length of such diffractive focussing devices is wavelength dependent. However, this can be corrected. We have tested optically
the efficiency of the chromatism correction on artificial sources (500 < λ < 750 nm): the images are diffraction limited, and the dynamic range measured on an artificial double source reaches 6.2
10 − 6. We have also validated numerical simulation algorithms for larger Fresnel interferometric arrays. These simulations yield
a dynamic range (rejection factor) close to 10 − 8 for arrays such as the 3.6 m one we propose. A dynamic range of 10 − 8 allows detection of objects at contrasts as high as than 10 − 9 in most of the field. The astrophysical applications cover many objects in the IR, visible an UV domains. Examples are presented,
taking advantage of the high angular resolution and dynamic range capabilities of this concept. 相似文献